A silicon wafer manufacturing method will be described as an example of conventional wafer manufacturing method. Silicon single crystal ingots are first grown by the Czochralski method (CZ method), etc., and sliced into silicon wafers. Then those silicon wafers are subjected to chamfering, lapping and etching steps in succession, after which the wafers undergo a polishing step where at least the wafer main surface is mirror-polished.
In the wafer polishing step, a double-side polishing apparatus is used occasionally when both front and back surfaces of silicon wafers are polished. As the double-side polishing apparatus, a so-called four-way double-side polishing apparatus is normally employed that has a planetary gear construction in which carrier plates for holding wafers are arranged between a sun gear provided at the center portion and an internal gear provided at the perimeter portion.
The four-way double-side polishing apparatus can simultaneously polish both front and back surfaces of silicon wafers by inserting silicon wafers into and holding them in a plurality of carrier plates on which wafer holding holes are formed, pressing upper and lower turn tables, in which polishing pads are attached to wafer-facing surfaces, against front and back surfaces of each wafer and rotating the turn tables in relative directions while supplying polishing slurry from above the held silicon wafers and by concurrently rotating and revolving the carrier plates using the sun and internal gears.
A double-side polishing apparatus as described in Japanese Patent Laid-open (Kokai) Publication No. 10-202511 is among other known forms of double-side polishing apparatuses. FIG. 5 illustrates a schematic sectional view of the double-side polishing apparatus. A double-side polishing apparatus 41 comprises a carrier plate 46 with a plurality of wafer holding holes in which silicon wafers 44 are held, upper and lower turn tables 42 and 43 which are arranged above and below the carrier plate 46 and to whose wafer-facing surfaces polishing pads 45 are attached for simultaneously polishing both front and back surfaces of the silicon wafers 44 and carrier motion means (not shown) for moving the carrier plate 46 sandwiched between the upper and lower turn table 42 and 43 within a plane parallel to the front surface of the carrier plate 46. The upper turn table 42 is provided with a cylinder 47 for applying rotation and polishing loads, a housing 48 for transmitting those loads to the upper turn table 42 and fixing means 49 such as bolts for fixing the housing 48 and the upper turn table 42. On the other hand, the lower turn table 43 is provided with the cylinder 47 for imparting rotation from a motor and a speed reducer (not shown) to the lower turn table 43 and a thrust bearing 50 that supports the load of the turn tables.
In such the double-side polishing apparatus 41, the carrier plate 46, sandwiched between the upper and lower turn tables 42 and 43, is caused to make a circular motion not accompanied by its rotation by the carrier motion means (not shown) via a carrier holder 51, that is, a type of swinging motion in which the carrier plate 46 circles without rotating while remaining eccentric by a given distance from the rotational axis of the upper and lower turn tables 42 and 43. At this time, the silicon wafers 44 are held in the wafer holding holes of the carrier plate 46 so as to be rotatable, allowing the silicon wafers 44 to turn together (rotate) in the direction of rotation of the faster rotating turn table as a result of rotation of the upper and lower turn tables at different speeds or in different directions on the rotational axis.
Therefore, when both front and back surfaces of silicon wafers are polished, it is possible to simultaneously and uniformly polish both front and back surfaces of silicon wafers by inserting the silicon wafers into respective wafer holding holes on the carrier plate, holding the silicon wafers therein and causing the carrier plate to make a circular motion not accompanied by its rotation while causing the wafers themselves to rotate in the wafer holding holes by rotating the upper and lower turn tables at different speeds or in different directions, with slurry containing polishing abrasive grain supplied to the silicon wafers. Double-side polishing apparatuses in such a form have lately come into frequent use with recent increase in wafer diameter since the apparatuses are capable of double-side polishing of large-sized wafers with ease.
However, if a plurality of batches of wafers are polished repeatedly using a four-way double-side polishing apparatus as described above or double-side polishing apparatus that polish wafers by causing the carrier plate to make a circular motion not accompanied by its rotation, the polishing ability and other factors of polishing pads attached to the upper and lower turn tables change over time due to polishing pads' life, clogging and other factors. For this reason, polishing of a plurality of batches of wafers without replacement of the polishing pads has given rise to change in polished wafer shape over time with more polished batches, resulting in wafer shape differences between batches and problems that it is impossible to maintain wafer quality stably.
To solve the problems, wafers have traditionally been polished by changing various polishing conditions according to changes in polishing ability of the polishing pads and other factors over time, thereby controlling change in wafer shape over time. For instance, there is a method for controlling wafer shape in which the turn table shape itself is changed by changing conditions such as turn table temperature.
Although turn tables thought to be preferred for use in a double-side polishing apparatus are those that do not deform during wafer polishing, if such turn tables not deforming during wafer polishing are used, it will be difficult to ensure a fit between wafers and turn tables under varying polishing conditions. Moreover, it will be impossible to control wafer shape in response to changes in polishing ability and other factors of polishing pads over time.
To avoid such difficulties associated with wafer shape control, turn tables are generally made of a material that deforms to a certain extent and particularly under varying temperatures, thus allowing wafer shape control by flowing cooling water, etc., through the turn tables and changing their temperature to vary the shape of the turn tables.
However, despite an attempt to control wafer shape by flowing cooling water, etc., through the turn tables and changing their temperature, the responsiveness of turn table deformation to varying turn table temperature (linearly relative to change in turn table temperature and the like) in conventional double-side polishing was poor, making it impossible to ensure high precision in the turn table shape control. Particularly if polishing conditions and the like are substantially changed during wafer polishing, there was caused a problem that it has been impossible to control the turn tables into a desired shape through the temperature control in the turn tables alone. When a plurality of batches of wafers are polished repeatedly, it has been difficult to control the turn table shape as desired with increasing number of wafer batches due to poor responsiveness of turn table deformation, making it impossible to control batch-by-batch wafer shape stably and with high precision. In particular, repeated polishing of a plurality of batches of large-sized wafers such as 300 mm-diameter wafers has often led to a convex wafer shape, thus exhibiting remarkably deteriation of a wafer shape such that flatness of GBIR (Global Back Ideal Range), etc. is deteriorated. In other words, conventional turn table temperature control alone has failed to sufficiently control time-varying wafer shape.